1. Field of the Invention
The invention generally relates to light emitting semiconductor devices such as semiconductor lasers and light emitting diodes. More specifically, the invention relates to confinement of a free carrier in the active region of the semiconductor device.
2. Related Technology
Vertical-Cavity Surface-Emitting Lasers (VCSELs), Surface Emitting Lasers (SELs) and Light Emitting Diodes (LEDs) are becoming increasingly important for a wide variety of applications including optical interconnection of integrated circuits, optical computing systems, optical recording and readout systems, and telecommunications.
VCSELs, SELs, and LEDs are generally formed as a semiconductor diode. A diode is formed from a junction between a p-type material and an n-type material. In VCSELs, the p-type semiconductor material is most often aluminum gallium arsenide (AlGaAs) doped with a material such as carbon that introduces free holes or positive charge carriers, while the n-type semiconductor materials is typically AlGaAs doped with a material such as silicon that introduces free electrons, or negative charge carriers.
The PN junction forms an active region. The active region typically includes a number of quantum wells. Free carriers in the form of holes and electrons are injected into the quantum wells when the PN junction is forward biased by an electrical current. At a sufficiently high bias current the injected minority carriers form a population inversion in the quantum wells that produces optical gain which is used inside a resonant cavity to cause lasing in semiconductor lasers such as VCSELs.
Free carriers that escape the quantum wells into the surrounding semiconductor and recombine there do not contribute to the optical gain and are parasitic currents which generate heat and reduce the efficiency of the light emitting device. This “carrier leakage” is one of the causes of the rollover of the light vs current curve. Current can only be increased so much and then light output reaches a maximum and drops off. Generally, higher temperatures result in lower maximum light output partially because the thermal energy of the carriers, electrons and holes, is increased allowing a larger fraction to contribute to carrier leakage. Electrical confinement in the active region can be particularly problematic in VCSEL devices, which tend to require high current densities for operation and is made worse in the highest frequency VCSELs where the highest current densities are used.